Starter/generator systems for engine applications may be required to deliver high torque to an engine when in a starting mode, and operate efficiently at other times in a DC or AC generator mode. In a typical engine starter/generator system 10, shown schematically in FIG. 1A, a starter/generator subsystem 2 is coupled to an engine 3 and provided with power from a battery 1. In order to produce a high level of torque in engine starting mode, the starter/generator 2 typically requires a relatively high input voltage. For example, a typical 28 VDC aircraft starter/generator subsystem for generating 100 foot-pounds of engine starting torque at speeds up to 20% of rated speed (e.g., 2400 revolutions per minute (rpm) for a 12,000 rpm drive system), requires a minimum of 30,000 watts input power. If the source of that power is a nominal 24 VDC aircraft battery, the required current can exceed 1,800 amperes, and the battery voltage will typically drop to 16 volts or less during the engine start transient. This high level of input current to the starter/generator subsystem presents a significant problem in the design of the aircraft electrical distribution system.
A conventional method for providing higher input current is to increase the voltage of the power sources used to support engine starting. For example, as shown schematically in FIG. 1B, multiple batteries 11, 12 (each nominally 24 VDC) for an aircraft starter/generator may be connected in series during the engine start transient. The voltage distribution buses in the aircraft then must be configured differently to support engine starting and normal generator operation (nominal 48 VDC voltage for starting and 28 VDC voltage for generating). This complicates the distribution system design and control, particularly when aircraft safety requirements are considered.
It also is desirable to implement the starter/generator subsystem 2 as part of a fault tolerant system, in which one or more starter/generator functions are configured with redundant or multiple elements. Fault tolerant system architectures (including use of multiple isolated windings, incorporating redundancy in a machine and/or power controller/converter functions, etc.) are described in more detail in U.S. Pat. No. 7,064,526 (assigned to the same assignee as the present disclosure), the entire disclosure of which is incorporated herein by reference.
There is a need for a starter/generator system which can produce high levels of starting torque even when the input voltage to the starter/generator is relatively low, and which can be used in a fault tolerant power system architecture.